Members of the 3rd-Generation Partnership Project (3GPP) are currently developing Release 9 improvements for the so-called Long-Term Evolution (LTE) wireless systems. As part of this work, the addition of a mechanism for downlink flow control is under consideration.
The motivation for downlink flow control is that a mobile station's demands for resources (e.g., processing power, memory, battery power) vary over time, depending, among other things, on the instantaneous data rates received and/or transmitted by the device. To keep the costs of mobile stations to reasonable levels, terminals are designed to handle the most common load conditions, rather than to handle the advertised peak rate over sustained periods. In the event that a wireless base station (an evolved Node-B, or eNB, in LTE systems) directs downlink data to a mobile station at its highest achievable rate for a long period of time, a terminal running short on resources may be forced to drop data elements.
The objective of the downlink flow control mechanism desired for Release 9 LTE is thus to provide means for the mobile station to signal back to the eNB a request for a reduction in downlink data transmission rate. The eNB, upon receipt of this signal, then lowers the rate at which it schedules data on the downlink traffic channel (e.g., LTE's DL-SCH) for a certain period.
Several possible approaches have been described in proposals submitted to the LTE working group. One approach is to implement flow control at the Medium Access Control (MAC) layer. With this approach, a flow control indication to the eNB is provided by the mobile station as part of the MAC protocol by, for example, using a MAC Control Element or a bit in a MAC Control protocol data unit (PDU) header. Of course, a new or modified MAC control message would be needed to implement this approach. Another approach utilizes Radio Link Control (RLC) flow control: an indication is provided by the mobile station as part of the RLC protocol. This proposed approach is similar to that employed in the UTRAN (UMTS Terrestrial Radio Access Network) RLC, where the receiver controls a transmission window of the transmitter to enforce flow control. However, this approach is not likely to be very effective. Because the feedback is in units of RLC sequence numbers and because the RLC PDU can be very large, attempts to limit downlink data flow to a small number of RLC PDUs may still result in excessively large data transfers.
Still another approach exploits the existing channel quality mechanisms for LTE. With this approach, code points to indicate flow control levels are reserved in the Channel Quality Indicator (CQI) transmissions made by the mobile station (in LTE, on the UL-SCH or on PUCCH). While this approach would be relatively easy to standardize, it adds overhead to the frequently signaled CQIs, even when flow control is not needed. It has also been proposed that mobile stations can use Radio Resource Control (RRC) procedures to modify the User Equipment (UE) capabilities signaled to the base station, to indicate its maximum sustainable transmission rate. Again, this approach adds overhead, and may also involve processing times that limit the responsiveness of this approach.
Finally, end-to-end flow control, using transport layer signaling (e.g., transport control protocol, or TCP) or application layer signaling (e.g., real-time protocol, or RTP) has been proposed. With this approach, flow control is performed end-to-end by the transport or the application protocol. While this approach requires no modification to the wireless standards, it also is generally not suitable for controlling data flow in real-time, since the wireless base station is not involved in the end-to-end flow control process.